The invention relates to a circuit device for operating a high pressure discharge lamp with successive current phases, which circuit device is provided with means for operating the connected lamp in a dimmed state by means of a dimming signal Ds, which circuit device comprises:
input terminals for connecting the circuit device to a power supply source,
output terminals for connecting the lamp to be operated,
a switch-mode power supply (smps) with a converter provided with a semiconductor switch for generating, at a converter output, a converter output voltage across buffer capacitor means and a converter output current, and
a control circuit for generating a control signal in dependence upon the dimming signal for controlling the semiconductor switch.
A circuit device of the type mentioned in the opening paragraph is known from U.S. Pat No. 5,828,178. In general, high-pressure discharge lamps are operated using successive current phases of periodically alternating polarity. In a frequently applied structure of the circuit device, the converter is connected, for this purpose, to a commutator, for example, in the form of a bridge circuit.
In the known circuit device, an optical sensor is provided for detecting light generated by the lamp. This enables a control for dimming the lamp to be realized.
The known dimming system for high-pressure discharge lamps, however, has a number of serious practical drawbacks. One serious drawback relates to the use of an optical sensor. On the one hand, because in order to properly detect the light generated by the lamp, correction for ambient light is required and, on the other hand, because such a detection is very sensitive to soiling of the sensor. An additional, frequently encountered problem relates to the fact that the lamp readily starts to flicker, which is visually very disturbing. Besides, there is a substantial risk that the lamp will cease burning during dimming or in the dimmed state. Another frequently encountered drawback relates to the fact that operation of the lamp in a dimmed state leads to blackening of the wall of the discharge vessel, causing the luminous flux of the lamp to decrease in the course of the service life of the lamp.
It is an object of the invention to provide a measure by means of which said drawbacks are counteracted.
To achieve this, a circuit device of the type mentioned in the opening paragraph is characterized, as a circuit device in accordance with the invention, in that the circuit device is provided with means for generating a converter output voltage-related signal Lvs to form the control signal.
The circuit device in accordance with the invention has the advantage that signal formation takes place on the basis of voltage measurement instead of optical registration. A further advantage is that registration of the value and duration of the converter output voltage appears to be very suitable for detecting the maximum permissible dimmed state at which the lamp is still stable in operation, by comparing the detected signal with a limit value. At the beginning of each current phase, the converter output voltage generally is pulse-shaped with a pulse height and a pulse width, followed by a plateau voltage. Thus, in a favorable embodiment, the signal Lvs serves to detect the pulse height. In a different embodiment, the signal Lvs is used to detect the pulse width.
In a further embodiment of a circuit device in accordance with the invention, the signal Lvs serves to detect the plateau voltage. In a first modification, detection of the difference in plateau voltage of the signal Lvs in successive current phases takes place. In another modification, detection of fluctuations in the plateau voltage of the signal Lvs within one current phase takes place.
In the operating state, the converter carries the converter output current at its output. In an advantageous embodiment in accordance with the invention, the circuit device comprises means for generating a converter output current-related signal Icu, which is intended to form the control signal. Surprisingly, it has been found that controlling the semiconductor switch of the converter on the basis of both the converter output voltage and the converter output current enables a connected high-pressure discharge lamp to be operated in a stable manner over a large dimming range. Preferably, the signals Lvs and Icu are used to form a power signal Sv, for example by multiplying the signals Lvs and Icu. The power signal Sv is subsequently compared to a reference power value which depends upon the dimming signal Ds, and the result of the comparison is used to form the control signal for controlling the semiconductor switch of the converter. Detection of the maximum permissible dimmed state can be advantageously realized by limiting the value of the power signal Sv immediately after the start of each current phase.
Periodic detection of the converter output voltage and the converter output current enables a software-based approach, resulting in a substantial degree of freedom regarding the practical realization of the hardware. Preferably, the control circuit comprises a programmable processor, which is used to carry out one of the above-mentioned functions. Said control circuit preferably comprises means for comparing the signal Lvs to a limit value. This can be advantageously carried out by means of the programmable processor.
If the switching device can suitably be used to operate a metal halide lamp having a ceramic wall, then the limit value for comparing the signal Lvs preferably corresponds to a crest factor of at most 1.5. If the switching device can suitably be used to operate a metal halide lamp having a quartz glass wall, then the limit value preferably corresponds to a crest factor of at most 1.6. The term xe2x80x9ccrest factorxe2x80x9d is to be taken to mean, in this description and in the claims, the ratio between the pulse height of the converter output voltage and the height of the plateau voltage of the converter output voltage. The term xe2x80x9cceramic wallxe2x80x9d is to be taken to mean in the claims, a wall of a light-transmitting, densely sintered metal oxide, such as aluminum oxide and YAG, and of a light-transmitting metal nitride, such as AIN.